456 



NATURE 



[October h, 1909 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 



expressed by liis correspondents. Neither can lie undertake 



to return, or to correspond with the writers of, rejected 



inanuscripl.s intended for this or any other part 0/ Nature. 



No notice is taken of anonymous communications.] 

 Magnetic Storms. 



In supplement to my letter in the last issue of Nature 

 I may add that if a solar outburst, acting in the way 

 supposed, causes a magnetic storm which lasts eight hours, 

 the effective influence of the whole group of electric streams 

 at the distance of the earth must extend over a breadth 

 of about six million miles ; so that if simply conical, with 

 vertex at the sun's centre, the angle of the cone would 

 be four degrees. Projected back to the surface of the 

 sun, this would correspond to what we may call a " spot " 

 about one-thirtieth of the visible disc in diameter ; but, 

 inasmuch as the trajectory of the particles in the beam 

 would be slightly curved, the size of the actual solar 

 eruption could be much less. 



Until the main outlines of the view advocated are 

 approved by those most competent to judge, it is useless 

 to enter into further details. 



I regret to notice a numerical slip — fortunately un- 

 important, since it affects nothing else — in the top line 

 of my letter on p. 426, where the current equivalent should 

 be expressed, not in hundred thousands, but in hundred 

 millions of amperes — an order of magnitude which is 

 " reasonable " rather than " surprisinglv moderate." 



October 9. ' Oliver Lodge. 



Magnetic Storms and Solar Eruptions. 

 I HAVE read Sir Oliver Lodge's letter (Nature, 

 October 7, p. 425) on the above subject with some surprise. 

 The emission theory which he supports, and which he 

 claims to have originated, regards kathode rays from the 

 sun as the electric carriers, and so is presumably the 

 same as has been actively advocated by Prof. Kr. Birke- 

 land for a number of years. He seems, however, to be 

 unaware of the existence of Birkeland's volumes ' on the 

 subject, and of the numerous numerical calculations therein 

 contained. He also makes no reference to the important 

 mathematical work of Prof. Stormer, which cannot, I 

 thmk, be disregarded by anyone whose theory postulates 

 the introduction of charged particles from without into 

 the earth's magnetic field. The general idea that magnetic 

 storms are due to some action arising in the sun goes 

 back to at least the time of Broun and Balfour Stewart, 

 and different forms of the emission theorv have naturally 

 presented themselves to various minds independently, as 

 Rdntgen, kathode, and other rays came successively under 

 our ken. It is when we come to details that real 

 troubles arise. Most people, I take it, have little 

 difficulty in believing in a general way that the changes 

 of declination experienced at a single magnetic observa- 

 tory say Kew, during a magnetic storm can be accounted 

 for by a stream of electrons in the magnetic meridian, 

 provided it is possible for the direction and intensity of 

 the stream to be altered at frequent intervals. One doubts 

 this just as little as that the motion of the magnet of the 

 declination magnetograph at Kew on September 25 

 could be reproduced with the aid of a copper wire, a single 

 battery cell, a commutator, and a resistance box. Those 

 who have seen Stormer's calculations and studied Birke- 

 land's volumes will realise, however, that to be re- 

 garded as an advance of knowledge at the present 

 day, a theory must afford an explanation, not merely 

 of what IS taking place in a single magnetic element 

 at a single station, but of what is taking place in 

 all three elements at a number of stations. Coming, now, 

 to Sir Oliver Lodge's own calculation, it seems based on 

 an inadequate idea of the phenomena of the late storm, 

 derived from a description of one or two of the more 

 striking changes at Kew as recorded in vour columns and 

 those of the Times. It is rare for a disturbance to be 

 imited to the declination, i.e. for the disturbing force to 

 be wholly perpendicular to the magnetic meridian. The 

 component in the magnetic meridian is, as a matter of 



1 ," ExDi^dit-on Norv^gienne de 1899-igoo," and " The Norwegian Aurora 

 Polaris Kxpedition, ;qo2-3," vol. i. 



NO. 2085, VOL. 81] 



fact, usually the larger. A vertical component is also 

 usually present. A magnetic storm does not usually 

 consist of a disturbing force in a fi.Ked or nearly fixed 

 direction, waxing and waning. Each of the three elements 

 usually exhibits values both above and below the normal, 

 and not infrequently there are many excursions on both 

 sides of the mean. This will, I think, be readily recog- 

 nised by anyone who consults the reproduction of the 

 Stonyhurst curve of September 25 in your columns and of 

 the Kew curves in the Electrician. After inspecting these 

 curves it will, I think, be recognised that it is quite out 

 of the question to limit the passage of the imaginary 

 solar jet, as Sir Oliver Lodge does, to the fifteen minutes 

 near the end of the storm, when there occurred the 

 prominent declination oscillation to which he has confined 

 his attention. Even whilst ,this oscillation took place, it 

 was far from representing the total disturbance. 

 Simultaneously with it, but partly overlapping, as is often 

 the case, there was a verv large change in progress in 

 the horizontal force. Those looking at the curves will, I 

 think, agree that if there was a jet such as Sir Oliver 

 Lodge supposes, its time of transit took, not fifteen 

 minutes, but at least nine hours. His estimate of the 

 diameter of the cone thus requires multiplication by 36, 

 with a consequent multiplication of the cross-section, if it 

 were circular, by 1296. Large as this may appear, the 

 jet theory requires it to be often exceeded, as the storm 

 of September 25 was an unusually short one. The average 

 duration of the storms in Mr. Maunder's Greenwich list, 

 from 1882 to igojj, was almost exactly thirty hours, so 

 that the cross-section of the average storm-jet would be 

 naturally fully 14,000 times that given by the calculation 

 in your columns. The really crucial thing is that the 

 magnetic disturbances which occur simultaneously at 

 different stations are inter-related. It is in accounting 

 satisfactorily for these inter-relations that Birkeland, who 

 has given years of thought to the subject, encounters his 

 main difficulties. 



In pointing out these facts, I am not expressing any 

 opinion for or against any or all of the emission theories. 

 What I think is really called for at the present moment is 

 a reservation of judgment as to theories, and a more 

 minute study and inter-comparison of the records from 

 different observatories with a mind as unbiased as possible 

 by preconceived ideas. C. Ciiree. 



October 9. 



Fireball in Sunshine. 



With the sun shining in a beautifully clear sky on 

 October 6, at about 9.40 a.m. a large meteorite passed 

 over central PJngland, and was well observed from many 

 widely distant stations. People noticed it in Norfolk, 

 Suffolk, Gloucester, Somerset, and other counties, but the 

 observations, owing to the absence of visible sky marks, 

 are not very definite. 



The meteor was brilliant; it had a slow motion, 

 traversing a long path in about four seconds, and it left 

 a luminous trail of short duration. An observer at Bristol 

 says it burst with rocket-like effect at the finish. The 

 meteor had a radiant in the south or south-east sky, but 

 the place is uncertain. At the time of the observatior» 

 Leo was on the meridian and Virgo and Bootes near. 



At Cottesbrook, Northamptonshire, a loud detonatior* 

 follow'ed the meteor in four minutes, w'hich corresponds to 

 a distance of fifty miles. At East Haddon, Holdenby, and 

 other small towns and villages north-west of Northampton 

 the noise of an explosion was heard, doors creaked, 

 windows rattled, and people ran out of their houses irt 

 terror, thinking that an earthqu.ake had occurred. The 

 final disruption of the meteor evidently took place over 

 the region ten or fifteen miles north-west of North- 

 ampton, and its direction of flight was from S.S.E., so 

 it must have passed over, or ncarlv over, London. 



Further observations will be exceedingly useful if they 

 are sufficiently exact to be utilised. W. F. Denning. 



The Mansfield Automatic Water Finder. 



Can any reader of N-iiture supply the names of the 

 leading scientists " who are stated to have " thoroughly 



investigated " this instrument " and vouch for the 



successful application of the invention "? 



